1. Field of the Invention
The present invention relates to an epitaxial growth method for compound semiconductor thin films, and more particularly to an epitaxial growth method for a compound semiconductor thin film, capable of forming a p-n junction by control of growth temperature.
2. Description of the Prior Art
Recently, growth of semiconductors with a hetero-junction structure, in particular, compound semiconductors, has been widely used by virtue of the development of thin film growth techniques and equipment and the development of molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) methods. Also, semiconductor devices using hetero-junction structures made of a variety of compound semiconductors have been widely used.
In the growth of such structures over a semiconductor substrate, it is necessary to grow a layer containing an n or p type impurity on a semiconductor substrate. By the growth of such a doped layer, a p-n junction structure, which is a heterojunction structure between p type layer and n type layer to be grown on the substrate, is formed. Such a hetero- junction structure has been widely used.
In this connection, the adjustment of the concentration of the impurity contained in the doped layer and the adjustment of the conductivity-type of the doped layer are important factors for determining the characteristic of the final semiconductor device. In this regard, it is known that the development of doping processes is commercially and technically important.
Recently, heterojunction bipolar transistors (HBTs) using III-V compound semiconductors have been getting popular by virtue of their superior characteristics expected in high-speed devices. In particular, the epitaxy of InGaAs with p type carriers is an important technique in the fabrication of InGaAs/InP heterojunction bipolar transistors.
In order to realize such heterojunction bipolar transistors operating at a high speed, a reduced base resistance should be used. To this end, a very large amount of impurity should be doped in the base layer of a transistor structure in the fabrication of the transistor structure.
However, beryllium (Be) or zinc (Zn) conventionally used as a p type impurity exhibits a very large diffusion coefficient at a high doping concentration. In this case, the performance of the semiconductor device is degraded.
In order to overcome such a problem, research on doping of a carbon impurity in an InGaAs is being made. Such a carbon impurity has superior characteristics in that it exhibits a low diffusion coefficient upon the growth of a GaAs and a high doping concentration.
Generally, the carbon impurity is known to have an n type conductivity in InGaAs. As a result of such research, it is also known that it is impossible to grow InGaAs with p type carriers using metalorganic compounds as a Group III source material. It is also known that such a p type InGaAs can be formed only when a source impurity material such as carbon tetrachloride (CCl.sub.4) is additionally supplied. Recently, it has also been reported that the p type InGaAs can be grown only when solid indium is used as a source material for providing indium (Reference: T. P. Chin et al, Highly Carbon-Doped P-Type GaInAs and GaInP by Carbon Tetrachloride in Gas Source Molecular beam Epitaxy, Appl. Phys. Lett., 59:2865, 1991).
Thus, the conventional techniques have a problem in that two different doping processes are required to form a p-n junction structure of InGaAs.